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Topic: What the effect of eluent on HPLC retention time?  (Read 8909 times)

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Offline Chaste

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What the effect of eluent on HPLC retention time?
« on: November 23, 2010, 11:30:02 PM »
Given nonpolar aromatic compounds were to be separated by eluent which consists of 65% methanol and water and then change to 90% methanol, what is its consequent effect in separation?

since the compounds are already non-polar, they wouldn't have interaction with mobile phase. My guess would be by altering the mobile phase to be less polar, the retention time are increased, as the solutes now interact with the mobile phase more?

Can someone enlighten me?

Offline Train

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Re: What the effect of eluent on HPLC retention time?
« Reply #1 on: November 24, 2010, 12:27:31 AM »
I agree with you that they would interact with the mobile phase more, but why would that lead to increased retention times?

Offline Chaste

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Re: What the effect of eluent on HPLC retention time?
« Reply #2 on: November 24, 2010, 12:31:10 AM »
I agree with you that they would interact with the mobile phase more, but why would that lead to increased retention times?

hmm.. so now there would be competing interactions of the analytes between the mobile and stat phase. retention time should decrease? as fewer non polar analytes are carried away by mobile phase?

Offline JGK

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Re: What the effect of eluent on HPLC retention time?
« Reply #3 on: November 24, 2010, 03:56:28 PM »
Given nonpolar aromatic compounds were to be separated by eluent which consists of 65% methanol and water and then change to 90% methanol, what is its consequent effect in separation?

since the compounds are already non-polar, they wouldn't have interaction with mobile phase. My guess would be by altering the mobile phase to be less polar, the retention time are increased, as the solutes now interact with the mobile phase more?

Can someone enlighten me?

The bit in bold is very wrong

Given that it's methanol and water, I'll assume that we are dealing with Reverse Phase Chromatography.

This probaly means you will have the most common stationary phase in the column, a C18 coated silica.

The retention time depends on the partition coefficient between the C18 phase and the mobile phase. Material which is less organic/more polar will elute faster while more organic material will be "held" (retained) by the stationary phase and will elute from the column more slowly.

Increasing the methanol component of the mobile phase reduces its polarity, which means that organic material will be less likely to be reatained on the non polar stationary phase and will therefore elute from the column more rapidly.
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Offline Chaste

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Re: What the effect of eluent on HPLC retention time?
« Reply #4 on: November 24, 2010, 06:55:42 PM »
alright thank you.

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